Account admin is the only person who can modify the logo.
The following steps is a complete guide of how to go about this.
1. Log in with your admin account.
2. Click on user button.
3. Click on "manage this organisations" on the list of options under the user button
4. Click on the button "Select a File" on the fourth option
"Change Logo: (Maximum size: 248px by 50px)"
5. Type of images supported are JPEG, PNG, GIF